Cherry Tree - ?fungus

we have a fairly large ornamental cherry tree that had a wonderful blossoming year last year but this year didn't seem too happy in early spring and now suddenly has lost 90% of its leaves and the remaining few leaves at the top have many holes in them.

From a quick look on the internet I think it might have "shot hole" - Can anyone give me some advice on how to treat it and how serious it is? 

We'd be very sad to loose it as it is a real feature in our garden. Many thanks for your help